This invention relates to a modulator assembly for use in a hydraulic vehicle braking system and is of the general type which includes valve means operable, in use, to isolate a pressure source from a brake actuator upon the occurrence of an incipient wheel skid and/or spin condition at a wheel braked by the actuator, and pressure modifying means operable to vary the pressure in the actuator in such a manner as to effect a correction of the wheel slip condition.
In one form of system of this general type for use in an anti-skid braking system, the pressure-modifying means is a de-booster device which is conveniently moved axially to vary said volume, being driven from a rotary armature of an electric motor by way of a screw mechanism. Actuation of the valve means, however, requires a sudden and rapid actuating impulse in one direction in order to produce a substantially instantaneous interruption of master cylinder pressure when a skid condition arises. Because of the incompatibility of these requirements, their implementation cannot readily be achieved without recourse to excessively complex and expensive mechanism.